This invention relates to a novel liquid crystal compound useful for display elements and a liquid crystal composition containing the same.
Display elements utilizing liquid crystals have been broadly used for watches, electric calculators, etc. These liquid crystal display elements utilize the optical anisotropy and dielectric anisotropy of liquid crystal substances, and the liquid crystal phases include nematic liquid crystal phase, smectic liquid crystal phase and cholesteric liquid crystal phase. However, among these, display elements utilizing nematic liquid crystals have been most broadly practically used. Namely, correspondingly to the electrooptical effect which has been applied to liquid crystal displays, display elements are classified into TN (twisted nematic) type, DS (dynamic scattering) type, guest-host type, DAP type, etc. As for liquid crystal substances used for the respective modes, those which exhibit liquid crystal phases within a temperature range as broad as possible are preferred. However, it is the present status that there is no single substance which satisfies the above conditions, but several kinds of liquid crystal substances are mixed with one another or with non-liquid crystal substances, for practical use. Further, the above substances are required to be stable to moisture, light, heat, air, etc. As to nematic liquid crystal compositions of TN mode among them, in order to drive the display elements using the compositions under a low voltage, those having a large positive dielectric anisotropy value (.DELTA..epsilon.) have been required. Also, in order to raise the response speed, it has been required that the viscosity be as low as possible.
Further, liquid crystal compositions having a large optical anisotropy value (hereinafter abbreviated to .DELTA.n) can inhibit occurrence of color unevenness due to a partially non-uniform distance between substrates of liquid crystal display elements, to thereby reduce the distance between the substrates, which results in an advantage that it is possible to increase the intensity of electric field even under the same impressed voltage. Thus compounds having a large .DELTA.n as a constituting component of liquid crystal compositions have been required.
The object of the present invention is to provide a novel liquid crystal compound which satisfies these requirements as much as possible, as a component constituting nematic liquid crystal compositions. SUMMARY OF THE INVENTION
The present invention resides in
a liquid crystal ester compound expressed by the formula ##STR5## wherein R.sup.1 represents an alkyl group or an alkoxy group each of 1 to 10 carbon atoms; ##STR6## represents ##STR7## Y represents --COO-- or --OCO--; any of ##STR8## n represents 0 or 1; and Z represents an alkyl group or an alkoxy group each of 1 to 10 carbon atoms, or a halogen atom of F, Cl or Br, and a liquid crystal composition containing at least one of these compounds.